During engine non-fueling conditions in which at least one intake valve and one exhaust valve are operating, such as deceleration fuel shut off (DFSO), ambient air may flow through engine cylinders and into the exhaust system. In some examples, an exhaust gas sensor may be utilized to determine ambient humidity during the engine non-fueling conditions. It may take a long time for the exhaust flow to be devoid of hydrocarbons during the engine non-fueling conditions, however, and, as such, an accurate indication of ambient humidity may be delayed. In one example, as shown in US 2014/0202135, an indication of ambient humidity may be determined based on a change in pumping current output by the oxygen sensor while modulating a reference voltage of the oxygen sensor between a lower first voltage (e.g., base voltage) and a higher second voltage. In this way, the oxygen sensor may be a variable voltage oxygen sensor capable of operating in a variable voltage mode.
However, the inventors have recognized that operating the oxygen sensor in the variable voltage mode may reduce the durability of the oxygen sensor. Specifically, operating the oxygen sensor at the higher second voltage may degrade the sensor, thereby reducing the longevity of the oxygen sensor. Operating the oxygen sensor in the variable voltage mode more frequently may increase a rate of degradation of the oxygen sensor.
In one example, the above issues may be addressed by a method for: applying to an oxygen sensor a lower first reference voltage to generate a first output and a higher second reference voltage to generate a second output during a first condition; applying the first reference voltage to the oxygen sensor to generate a third output during a second condition; and adjusting engine operation based on ambient humidity estimated based on the first, second, and third output. In this way, the oxygen sensor may only be operated in variable voltage mode during the first condition and oxygen sensor durability may be increased.
In one example, during selected conditions, the oxygen sensor is operated to determine an oxygen sensor reading corrected for dry air conditions. For example, during conditions when purge and crankcase ventilation gases are not ingested in an engine intake manifold, the reference voltage of an intake oxygen sensor may be modulated. Alternatively, in embodiments where the oxygen sensor is an exhaust oxygen sensor, the selected conditions may include engine non-fueling conditions, such as a deceleration fuel shut-off (DFSO) event. Specifically, the reference voltage of the oxygen sensor may be raised from a first, lower voltage where the output (e.g., pumping current) is representative of an oxygen reading in humid conditions, to a second, higher voltage where the output (e.g., pumping current) is representative of an increase in oxygen due to the full dissociation of humidity. A dry air pumping current may then be determined based on a ratio between the first output and the second output, the dry air pumping current indicative of an oxygen reading in dry air. The dry air oxygen reading (the ratio between the first and second output) may only be determined and updated periodically, such as following each engine start or after a duration of engine operation. Then, during engine operation when the selected conditions are met, the oxygen sensor may be operated at the first, lower voltage (and not the second, higher voltage). Ambient humidity may then be determined based on the previously determined dry air oxygen reading and the output of the oxygen sensor when operating only at the first, lower voltage. As such, multiple ambient humidity estimates may be taken while not operating the oxygen sensor in the variable voltage mode. A controller may then adjust engine operation based on the multiple ambient humidity estimates. By determining humidity based on a first oxygen sensor output at the lower, first voltage and comparing it to a dry air oxygen reading that is determined less frequently than the first oxygen sensor output, an amount of time in which the oxygen sensor operates in variable voltage mode may be reduced. As a result, degradation of the oxygen sensor may decrease while longevity of the sensor may increase.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.